1. Field of the Invention
This invention relates to methods and apparatus for dewaxing distillate and deasphalted residual hydrocarbon liquids. More particularly, it relates to a method and apparatus for dewaxing distillate and deasphalted residual hydrocarbon liquids comprising multistage dewaxing and separation of a vapor stream between stages. It may also include catalytic oligomerization of the separated vapor stream to convert olefins in the vapor stream to gasoline, distillate and lube boiling range materials, and downstream units to hydrofinish and recover lubes.
2. Discussion of Prior Art
The dewaxing of hydrocarbons to liquids of lower pour point is a process of great commercial significance. Although alternatives exist, it is now evident that the use of shape-selective catalysts, such as ZSM-5 type catalysts, to selectively convert paraffins, which contribute the most to high pour points, to smaller chain molecules, has many advantages over other methods. Thus, catalytic dewaxing over shape-selective zeolites will likely be the most commercially significant dewaxing process in the hydrocarbon processing industry.
Catalytic dewaxing of hydrocarbon oils to reduce the temperature at which precipitation of waxy hydrocarbons occurs is a known process and is described, for example, in the Oil and Gas Journal, Jan. 6, 1975, pages 69-73. A number of patents have also described catalytic dewaxing processes. For example, U.S. Pat. No. Re. 28,398 describes a process for catalytic dewaxing with a catalyst comprising a zeolite of the ZSM-5 type and a hydrogenation/dehydrogenation component. U.S. Pat. No. 3,956,102 describes a process for hydrodewaxing a gas oil with a ZSM-5 type catalyst. U.S. Pat. No. 4,100,056 discloses that a Mordenite catalyst containing a Group VI or Group VIII metal may be used to dewax a distillate obtained from a waxy crude. U.S. Pat. No. 3,755,138 describes a process for mild solvent dewaxing to remove high quality wax from a lube stock, which is then catalytically dewaxed to specification pour point.
Catalytic dewaxing processes may be followed by other processing steps such as hydrodesulfurization and denitrogenation in order to improve the qualities of the product. For example, U.S. Pat. No 3,668,113 describes a catalytic dewaxing process employing a Mordenite dewaxing catalyst which is followed by a catalytic hydrodesulfurization step over an alumina-based catalyst. U.S. Pat. No. 4,400,265 describes a catalytic dewaxing/hyrodewaxing process using a ZSM-5 type catalyst wherein gas oil is catalytically dewaxed followed by hydrodesulfurization in a cascade system.
In catalytic dewaxing processes using shape-selective catalysts, such as a ZSM-5 type catalyst, the waxy components, particularly the n-paraffins, are cracked by the zeolite into lighter products containing olefinic material. However, these lighter products have a boiling point (b.p.) predominantly 610.degree. F..sup.- (321.degree. C..sup.-). Olefinic fragments are unstable to oxidation; so the dewaxed oil may be subsequently hydrogenated over catalysts to saturate the olefins and improve the oxidation stability of the oil. The hydrogenation catalysts generally used are mild hydrogenation catalysts, such as a CoMo/Al.sub.2 O.sub.3 type. The color of the oil may also be improved in this hydrofinishing process.
U.S. Pat. No 4,428,819 to Shu et al discloses a process for hydrofinishing a catalytically dewaxed oil in which the residual wax content of the dewaxed oil is isomerized over a hydroisomerization catalyst. Typically, heavier lube fractions (greater than 610.degree. F. (321.degree. C.) b.p.) contain waxy components comprising normal paraffins, branched paraffins and cyclo paraffins. When a shape-selective catalyst, such as HZSM-5, is used to dewax these feeds, the normal paraffins crack much faster than the branched paraffins and cycloparaffins. HZSM-5 is a form of ZSM-5, with only hydrogen attached to the active sites, that is, no metals added.
Experience with ZSM-5 based catalytic dewaxing has shown that the dewaxing of higher boiling point or residual feeds (greater than 610.degree. F. b.p.) pose significantly greater problems than those experienced with lighter distillate feeds. Catalyst aging is particularly a problem for heavier feeds which cause catalysts to display a more rapid loss of activity relative to a fixed pour point specification 20.degree. F. (e.g., -7.degree. C.). This loss of activity results in higher aging rates (10.degree. F./day by which the operating temperature of a dewaxing catalyst must increase to maintain desired conversion) and lower on-stream capacity factors. Loss of catalyst activity, however, is also experienced in dewaxing lighter distillate feeds.
It would be desirable to find a way to prolong hydrodewaxing catalyst activity at low cost.
Recent developments in zeolite catalysts and hydrocarbon conversion methods and apparatuses have also created interest in utilizing olefinic feedstocks for producing heavier hydrocarbons, such as C.sub.5.sup.+ gasoline, distillates or lubes. These developments have contributed to the development of the Mobil olefins to gasoline/distillate (MOGD) method and apparatus, and the development of the Mobil olefins to gasoline/distillate/lubes (MOGDL) method and apparatus.
In MOGD and MOGDL, olefins are catalytically converted to heavier hydrocarbons by catalytic oligomerization using an acid crystalline zeolite, such as a ZSM-5 type catalyst. Process conditions can be varied to favor the formation of either gasoline, distillate or lube range products. U.S. Pat. Nos. 3,960,978 and 4,021,502 to Plank et al disclose the conversion of C.sub.2 -C.sub.5 olefins, alone or in combination with paraffinic components, into higher hydrocarbons over a crystalline zeolite catalyst. U.S. Pat. Nos. 4,150,062; 4,211,640; and 4,227,992 to Garwood et al have contributed improved processing techniques to the MOGD system. U.S. Pat. No. 4,456,781 to Marsh et al has also disclosed improved processing techniques for the MOGD system. U.S. Pat. No. 4,433,185 to Tabak teaches conversion of olefins in a two-stsge system over a ZSM-5 or ZSM-11 type zeolite catalyst to form gasoline or distillate.
Olefinic feedstocks may be obtained from various sources, including from fossil fuel processing streams, such as gas separation units, from the cracking of C.sub.2.sup.+ hydrocarbons, such as LPG (liquified petroleum gas), from coal by-products and from various synthetic fuel processing streams. U.S. Pat. No. 4,100,218 to Chen et al teaches thermal cracking of ethane to ethylene, with subsequent conversion of ethylene to LPG and gasoline over a ZSM-5 type zeolite catalyst.
The conversion of olefins in a MOGDL system may occur in a gasoline mode and/or a distillate/lube mode. In the gasoline mode, the olefins are catalytically oligomerized at temperatures ranging from 400.degree. to 800.degree. F. and pressures ranging from 10 to 1000 psia. To avoid excessive temperatures in an exothermic reactor, the olefinic feed may be diluted. In the gasoline mode, the diluent may comprise light hydrocarbons, such as C.sub.3 -C.sub.4, from the feedstock and/or recycled from debutanized oligomerized product. In the distillate/lube mode, olefins are catalytically oligomerized to distillate at temperatures ranging from 350.degree. to 600.degree. F. and pressures ranging from 100 to 3000 psig. The distillate is then upgraded by hydrotreating and separating the hydrotreated distillate to recover lubes.
Although lubes can be produced from olefins by the prior art methods, MOGDL and catalytic hydrodewaxing were not combined because the types (by b.p. distribution) of olefins in hydrodewaxed stock were not known so the hydrodewaxed stock was hydrotreated to saturate any olefins rather than recover them.